Traditionally, phone calls have been made over circuit-switched (CS) networks in which a reliable connection for a call is created between the communication end points. The circuit-switched connection (i.e. the “circuit”) provides a channel in the form of a specified, predetermined path for routing the voice information of the call. Because the required network resources are thus reserved for the call, there is a low probability of information being lost unless there is actually a network fault. Furthermore, the delay per unit information is constant. For these reasons the connection can be considered reliable. However, circuit-switched networks typically only support a limited bandwidth, i.e. only a limited range of frequencies from the voice signal can be encoded for transport over the circuit switched network.
More recently, voice over internet protocol (VoIP) has become a widely used alternative to circuit-switched calling, with advantages in cost and quality, but suffering from lower reliability. To transmit using VoIP (or indeed any such packet-switched medium), the encoded voice data of the call is divided into a plurality packets and the relevant destination address is added into a header of each packet. The route for each packet is then determined “on the fly” by the routing equipment of the packet-switched network. The packet-switched channel may therefore be considered a “virtual” channel. That is, in contrast with a circuit-switched channel, the physical path for each packet is not fixed or reserved for the call, and different packets of the same call may be routed differently without any predetermined path. Instead, the channel exists in the sense that the end points have performed a handshaking procedure in order to establish a session between them. The fact that the voice encoding is not a fixed feature of the network means that packet-switched calls allow a larger frequency range to be encoded and therefore potentially better call quality. On the other hand, the packet-based routing is less reliable because a certain amount of packet loss is expected (i.e. that expectation is an intrinsic feature of the system, e.g. with overloaded routers being arranged to discard packets, and error detection algorithms at routers or endpoints being arranged to reject corrupted packets). Furthermore, different packets can be delayed by different amounts, leading to further reliability issues for real-time communications.
The VoLGA (“Voice over LTE via Generic Access”) forum provides specifications for implementing mobile VoIP calls using Long-Term Evolution (LTE) technology. VoLGA specifies the use of a “fall-back” mechanism which resorts to a circuit-switched channel when conditions over the packet-switched channel are poor.